The Hyperloop is a pressurized tube system in which passenger cars zoom around on an air cushion, at up to 800 miles an hour.

Musk envisions the tubes sitting on pylons, some of them raised over existing highways between the two cities, presumably so that the Luddites who still want to drive can see what they're missing as Musk's Hyperloop pods whoosh past.

That doesn't factor in the cost of buying the land needed to build the Hyperloop track, the Times notes:

Michael L. Anderson, an associate professor of agricultural and resource economics at the University of California, Berkeley, predicted that the cost of the entire project would be closer to $100 billion.

That's because the government would need to buy up to 1,100 different parcels of land just to get the space to build the thing, the LA Times noticed in an article about high-speed rail between the two cities. Musk envisions that the land cost would be reduced because the Hyperloop would travel in the median of I-5, which the government already owns. But still — the bill will be higher than Musk estimates.